1. Field of the Invention
The invention relates to a process for the direct synthesis of organochlorosilanes using CuCl prepared from the copper-containing residues from the direct synthesis of organochlorosilanes.
2. Background Art
In the so-called "direct synthesis" of organochlorosilanes, in particular methylchlorosilanies, metallic silicon is reacted with chloromethane in the presence of copper catalysts, optionally in the presence of further cocatalysts and promoters. When the direct synthesis is performed industrially in fluidized-bed reactors, fine fractions of silicon and copper catalyst, contaminated by carbon particles and by various metal compounds from the secondary constituents derived from the technical-grade silicon raw material, are discharged together with the crude silane reaction product and unreacted chloromethane, and separated therefrom by downstream separation units, for example, cyclones. Furthermore, a reactor residue comprising silicon, catalyst, and metal halides is formed, and is discharged continuously or batchwise from the fluidized-bed reactors. Drying and filtration processes also result in the formation of contaminated fine dusts.
Processes for the recovery of the copper from reactor residue and fine dusts are known. For example, U.S. Pat. No. 5,306,328 discloses that the copper content of residues from the synthesis of methylchlorosilane can be converted into soluble copper(II) salts. These copper(II) salts can be reduced to copper metal using iron powder. The disadvantage of this process is that iron and copper must be employed at least in a molar ratio of 1:1, and that if copper is not employed as a catalyst in metallic form, further processing is necessary.
Processes for the preparation of CuCl from aqueous solutions are known. In these processes, pure Cu(II) compounds or so-called "metallic waste copper" are preferably used as sources of copper, and dissolved in mineral acids such as hydrochloric acid. Hydrochloric acid-containing CuCl.sub.2 solutions formed in etching processes for the production of circuit boards are also used. Solutions of this type, compared with the solutions produced in aqueous work-up of the process residues from the direct synthesis, have a relatively low content of additional impurities, for example, metal chlorides. After filtration, the copper-containing solutions are reduced to Cu(I) compounds with the aid of a suitable reducing agent, such as, for example, Cu, Fe, Zn, Al, hydroxylamine, sulfurous acid or SO.sub.2. During this process, it is ensured that the CuCl formed remains in dissolved form through addition of complexing agents such as NaCl and/or hydrochloric acid. After re-filtration, the solubility is then reduced to below the limit for CuCl by suitable measures, so that the solid CuCl at least partially precipitates. This can be achieved, for example, by diluting the solution massively with water. The solid CuCl is separated by filtration, washed, and dried. This process is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Volume A7, page 574.
The disadvantages of the foregoing process are that, first, due to the heavy dilution, relatively large reaction volumes and amounts of water are necessary, second, the resulting filtrates have to be worked up or suitably disposed; third, that an additional filtration step is necessary; and finally, relatively pure Cu(II) solutions are required as starting materials.
DE 901889 discloses that copper(I) chloride can be prepared in a yield of 85-90% from a copper(II) chloride solution with the aid of a reducing agent such as sulfur dioxide, sodium bisulfite or sodium sulfite. U.S. Pat. No. 4,758,352 discloses that the copper(I) chloride in the process described in DE 901889 is not separated quantitatively. The reason for the non-quantitative separation is given in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A7, page 574, in which it is shown that with increasing chloride ion concentration in the solution, the solubility of CuCl likewise increases considerably.